When Natsumi Mizuki's parents leave for London, her Aunt Izumi steps in to care for her. However, Izumi, an aspiring manga writer, is too preoccupied with her career to handle the unexpected arrival of a baby named Mirai, who happens to be a teenage version of Natsumi. As Mirai mysteriously time-travels to the past, young Natsumi must navigate the challenges of raising her while concealing her existence from the world. With the aid of futuristic baby-care gadgets and Izumi's questionable assistance, Natsumi struggles to maintain her 4th grade routine amidst this extraordinary situation.